This invention relates to controlled current inverter-induction machine drive systems, and more specifically to a method, and associated apparatus, for operating a plurality of arbitrarily loaded induction machines from a single, controlled current inverter.
The ruggedness and reliability of the induction machine has led to its widespread use in many industrial applications, including large electric and diesel-electric excavating equipment. Usually, in such industrial applications, the induction machine is energized from a single-frequency potential source, such as a phase controlled rectifier for example, through an inverter comprised of a plurality of pairs of switching devices, with switching devices of each pair coupled in series-aiding fashion and each of the pairs of serially connected switching devices coupled across the single frequency potential source. By regulating the duration and frequency of inverter switching device conduction, the amplitude and frequency, respectively, of inverter output current and hence, induction machine torque and speed, respectively, can be controlled accordingly.
Inverters employed to condition induction machine voltage in the manner described above are commonly designated either as controlled current source inverters or controlled voltage source inverters depending on whether inverter input current or inverter input voltage, respectively, is ultimately controlled responsive to operator commands. The controlled current source inverter, abbreviated CCI, is particularly well suited for energizing alternating current machines constrained to provide constant torque over a fixed frequency range. In addition, in comparison with controlled voltage source inverters, the controlled current source inverter is less complex and less expensive to manufacture, thus making the controlled current source inverter an attractive alternative to conventional controlled voltage source inverters for many applications.
For the sake of economy, it is often desirable to operate several induction machines from a single inverter. While controlled-voltage source inverters are capable of operating several induction machines without difficulty, the same is not true of controlled-current source inverters. If a plurality of machines are operated from a controlled-current source inverter, and the machines are not equally geared, equally loaded or coupled to a common shaft, then there is a great probability that the most heavily loaded machine will quickly become under-excited and pull out. Should one of the induction machines become under excited and pull out, inverter instability would likely result, possibly causing damage to the inverter.
One proposed solution to the problem of operating a plurality of arbitrarily loaded induction machines from a controlled current source inverter has been to substitute the combination of a DC-DC chopper, energized from a DC source, and a voltage regulator, in place of the conventional alternating current-to-direct current phase controlled rectifier. The voltage regulator controls the chopping frequency of the DC-DC chopper, causing it to chop at a relatively high rate so that the combined chopper-voltage regulator-inverter circuit appears electrically the same to the induction machine as a conventional controlled voltage source inverter. However, to achieve satisfactory dynamic machine drive system performance, the DC-DC chopper must be capable of operation at relatively high frequencies, thus requiring that the chopper circuit be fabricated of costly, high current, switching devices.
In contrast, the present invention concerns a method and apparatus, compatible with conventional controlled current source inverters, for operating a plurality of parallel-coupled, arbitrarily loaded induction machines from a single controlled current source inverter.
It is an object of the present invention to provide a method and apparatus for operating a plurality or arbitrarily loaded induction machines from a single controlled current inverter source which is compatible with present day inverters.
It is another object of the present invention to provide a method and apparatus for operating a plurality of arbitrarily loaded induction machines so that machine pull-out and resultant inverter instability are avoided.